As a method of protecting copyrights upon distributing digital data such as image data, audio data, and the like on the Internet, digital watermarking is receiving a lot of attention. Digital watermarking is a technique for embedding watermark information in digital data such as image data, audio data, and the like so as not to be perceived by a person. For example, as digital watermarking techniques for multi-valued images, various methods that exploit the redundancy of the density of a multi-valued image are known.
On the other hand, a binary image such as a document image has less redundancy, and it is difficult for such image to implement digital watermarking. However, some digital watermarking methods that utilize unique features of document images are known. For example, a method of shifting the baseline of a row (Japanese Patent No. 3,136,061), a method of manipulating an inter-word space length (U.S. Pat. No. 6,086,706, Japanese Patent Laid-Open No. 9-186603 (to be referred to as “reference 1” hereinafter)), a method of manipulating an inter-character space length (King Mongkut University, “Electronic document data hiding technique using inter-character space”, The 1998 IEEE Asia-Pacific Conf. On Circuits and Systems, 1998, pp. 419–422 (to be referred to as “reference 2” hereinafter), a method of handling a document image as a bitmap image expressed by black and white, two values (Japanese Patent Laid-Open No. 11-234502), and the like are known.
In the digital watermarking methods for a document described in references 1 and 2, the space length between neighboring words or characters is changed to express digital watermark information, and 1-bit information (1 or 0) is assigned in accordance with the size relationship of two space lengths.
Reference 1 specifies the use of an inter-word space length, and English and European documents as objects which are to undergo watermarking. However, by replacing the relationship between the space length and embedding information used in reference 1 by the relationship between the inter-character space length of a Japanese document and embedding information, the method of reference 1 can be applied to Japanese documents.
Reference 2 uses Thai in experiments, but does not particularly limit objects to which watermarking is to be applied. Hence, the method itself proposed by reference 2 can be applied to Japanese as in reference 1.
However, with the aforementioned conventional digital watermarking method of embedding watermark information in a document by manipulating the inter-character space length, the balance among characters in a document becomes unnatural depending on positions in the document where watermark information is embedded. FIG. 9 shows the first application result example in which the conventional digital watermarking technique is applied to text of a Japanese document. As shown in FIG. 9, a character “” after a punctuation mark is separated from the next character “” due to the presence of a space on the right side of the character “”, resulting in an unnatural balance of the document.
FIG. 10 shows the second application result example in which the conventional digital watermarking technique is applied to text of a Japanese document. In this case, the position of a punctuation mark “” is incorrect, resulting in an unnatural balance of the document. That is, since the space lengths before and after a character where watermark information has been embedded have a large difference, that portion readily looks unnatural.